1. Field of the Invention
This invention relates to a surface treatment method, which comprises forming a white or grayish white substance in micropores of the anodically oxidized film of aluminum or aluminum alloy.
2. Description of the Prior Art
In the prior art, aluminum and aluminum alloys have been widely used as construction materials, nameplates, vehicles, decorative materials, etc., with coloration according to various materials. Their color tones have been, for example, metallic color tones in construction materials, giving cold impressions, such as amber color, gold color, and silver color. Therefore, aluminum and alluminum alloys in the field of construction materials which are of warm pastel color tone on a base of opaque white color or grayish white color have been desired.
For realizing a pastel color tone, it is necessary to obtain a base color of opaque white color or grayish white color, and for this purpose there have been proposed several methods of surface treatment for opaque whitening.
That is, examples of methods for forming opaque white anodically oxidized films are as follows.
(1) The method in which an aluminum material is subjected to whitening on its surface by alternating current electrolysis, direct-current electrolysis, or dipping (as disclosed in Japanese Patent Publications No. 28921/1965 and No. 1523/1966);
(2) The method in which an opaque white anodically oxidized film is obtained according to a primary electrolytic method (e.g., the Ematal method as disclosed in Japanese Patent Publications No. 28147/1979 and No. 28148/1979);
(3) The method, in which, after application of an anodically oxidized film, secondary electrolysis is carried out to obtain an opaque white anodically oxidized film (as disclosed in Japanese Patent Publications No. 14519/1960 and No.11248/1979, and Japanese Laid-open Patent Application No. 37631/1975).
However, these methods involve the following problems. The chemical reagent employed is expensive or is a toxic substance, or its solution is unstable or requires a high bath voltage. Also, the extent of opaque whitening obtained is insufficient in practical application. Thus, under the present circumstances, coating methods are actually employed in place of these methods.